- Head to your WordPress admin console.
- Click Appearance. Editor. Select the Stylesheet (custom.css) file on the right hand menu (usually you will need to scroll down).
- Search (ctrl+f) for: font-family
- Change the font from the existing to your desired font. Try Helvetica.
- Click on update file.
- Refresh (F5) your webpage.
It goes without saying that you should backup custom.css before making any edits.